Directional coupler having directivity improving means situated near end of couplingregion



April 18, 1967 G. WOOLLEY. JR 3,315,182

DIRECTIONAL COUI LER HAVING DIRECTIVITY IMPROVING MEANS SITUATED NEAR END OF COUPLING REGION Filed Feb. 2, 1965 2 Sheets-Sheet l INVENTOR BRADFORD G. WOOLLEY, JR.

BY ac wk ATTORNEY A ril 18, 1967 B. G. WOOLLEY, JR 3,315,182 DIRECTIONAL COUPLER HAVING DIRECTIVITY IMPROVING MEANS SITUATED NEAR END OF COUPLING REGION 2 Sheets-Sheet 2 Filed Feb. 2, 1965 INVENTOR I lgure 5 BRADFORD G. WOOLEY, JR.

BY ac svwwk ATTORNEY United States Patent 3,315,182 DIRECTIONAL COUPLER HAVING DIRECTIVITY IMPROVING MEANS SITUATED NEAR END OF COUPLING REGION Bradford G. Woolley, Jr., Mountain View, Calif., assignor to Hewlett-Packard Company, Palo Alto, Calif., a corporation of California Filed Feb. 2, 1965, Ser. No. 429,772 12 Claims. (Cl. 3331-0-10) This invention relates to directional couplers, and particularly to high directivity TEM mode couplers.

TEM mode couplers are devices in which the combined effects of electric and magnetic fields couple power from one transmission line to an adjacent transmission line in a directional manner. Thus, if an incident electromagnetic wave propagates along the primary transmission line of a TEM mode coupler, another electromagnetic wave is directionally coupled to the parallel portion of the auxiliary transmission line. The coupling between the parallel portions of the primary and auxiliary transmission lines is inherently directional because the electric field and magnetic field couplings cancel each other in one direction. However, where the auxiliary transmission line diverges at an angle 0 from the primary transmission line, the magnetic field coupling is decreased by the cosine 0 more than the electric field coupling and an electromagnetic wave is nondirectionally coupled to the diverging portion of the auxiliary transmission line. This nondirectional electromagnetic wave has sufficient voltage to seriously impair the directivity of the TEM mode coupler at frequencies ranging above one kilo-megacycle per sec ond.

One method of reducing the voltage of this non-directional electromagnetic wave and thereby increasing the directivity of the TEM mode coupler is to place a conductive element near the auxiliary transmission line where it diverges from the primary transmission line as disclosed and claimed in Harmons co-pending patent application Ser. No. 301,450 entitled, High Directivity TEM Mode Coupler, filed August 12, 1963, and issued August 31, 1965, as US. Patent 3,204,206. The effect of such placement of the conductive element is to reduce the electric field coupling between the diverging portions of the primary and auxiliary transmission lines by grounding a portion of the electric field at the point of divergence. This compensates for the otherwise greater reduction in the magnetic field coupling between the diverging portions of the primary and auxiliary transmission lines and diminishes the voltage of the resultant non-directional electromagnetic wave. However, one disadvantage of this method is that the placement of the conductive element is critical and some tuning is required for best results.

It is, therefore, a general object of this invention to provide a TEM mode coupler having high directivity over a broad frequency range without the critical placement and adjustment of a conductive element.

Another object of this invention is to provide a high directivity TEM mode coupler for use at frequencies above, for example, four kilomegacycles per second.

In accordance with the illustrated embodiments of this invention, there is provided a high directivity TEM mode coupler having a distributed compensation structure disposed in the coupling region between the diverging portions of the primary and auxiliary transmission lines. In the case of an air-filled TEM mode coupler this distributed compensation structure comprises a plurality of elongated cavities in the inner surface of one or both ground planes of the TEM mode coupler. Alternatively, it comprises at least one dielectric element which is positioned on the inner surface of one or both ground planes and which does not project substantially into the region directly between the adjacent edges of the diverging primary and auxiliary transmission lines. In the case of a TEM mode coupler wherein the primary and auxiliary transmission lines are mounted in a dielectric slab, the distributed compensation structure comprises at least one cavity in the dielectric slab in the region directly between the adjacent edges of the diverging primary and auxiliary transmission lines.

Other and incidental objects of this invention will be apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIGURE 1 is .a cut-away perspective view of a high directivity coaxial coupler including a distributed compensation structure according to one embodiment of this invention;

FIGURE 2 is a top view in cross section of the high directivity coaxial coupler of FIGURE 1 including two distributed compensation structures according to other embodiments of this invention;

FIGURE 3 is a perspective view of one of the distributed compensation structures shown in FIGURE 2;

FIGURE 4 is a partially broken perspective view of a high directivity strip line directional coupler including a distributed compensation structure according to a further embodiment of this invention; and

FIGURE 5 is a top View in cross section of the high directivity strip line coupler of FIGURE 4 including distributed compensation structures according to still another embodiment of this invention.

Referring now to FIGURE 1, there is shown an airfilled coaxial coupler having primary and auxiliary conductors 12 and 14. These primary and auxiliary conductors 12 and 14 are positioned in the same plane in transverse coupling proximity between parallel conductive ground planes 16 and 18 and are supported by coaxial connectors 19 mounted in conductive side wall 20. Conductive ground planes 16 and 18 and conductive side wall 20 form an outer conductor which is connected to the outer conductors of coaxial connectors 19. The primary conductor 12 extends longitudinally between and is connected to the inner conductors of the coaxial connectors 19 mounted in the opposite end sections of conductive side wall 20. A central portion of the auxiliary conductor 14 is positioned adjacent and parallel to the primary conductor 12. One end portion of auxiliary conductor 14 diverges from the primary conductor 12 and is connected to the inner conductor of the coaxial connector 19 mounted in the side of conductive side wall 20. The other end portion of auxiliary conductor 14 also diverges from the primary conductor 12 and is connected to ground plane 18 through a load 24 (shown in FIGURE 2).

A distributed compensation structure comprising a plurality of elongated transverse cavities 26 is disposed in the inner surface of ground plane 18 beneath a parallel reference plane defined between the adjacent edges of the exposed and diverging portions of primary and auxiliary conductors 12 and 14. The distributed compensation structure is symmetrically positioned with respect to this reference plane as indicated by the dashed projection 28 of the reference plane on ground plane 18. Though the distance 30 between the outer edges of the distributed compensation structure and the projection 28 of the reference plane should not generally exceed one-tenth of the diameter or width of the primary or auxiliary conductors 12 or 14, the overall dimensions of the distributed compensation structure are not subject to critical tolerances. In particular the elongated transverse cavities 26 have a depth of two to fifteen one-thousandths of an inch, a width of one to ten one-thousandths of an inch, and a length which increases as the distance between the di- Ierging portions of the primary and auxiliary conductors l2 and 14 increases.

To achieve the greatest improvement in the directivity 3f the coaxial coupler 10, a similar distributed compensation structure is disposed in the inner surface of ground plane 16 adjacent to and above the reference plane defined between the adjacent edges of the exposed and diverging portions of primary and auxiliary conductors 12 and 14. In addition a pair of similar distributed compensation structures is disposed in the inner surfaces of ground planes 16 and 18 on either side of the reference plane defined between the adjacent edges of the unexposed and diverging portions of primary and auxiliary conductors 12 and 14.

These distributed compensation structures have the etfect of increasing the mutual inductance and, hence, the magnetic field coupling between the diverging portions of primary and auxiliary conductors 12 and 14. This compensates for the otherwise greater reduction in magnetic field coupling relative to the electric field coupling caused by the divergence of primary and auxiliary conductors 12 and 14 and substantially reduces the voltage of the resultant nondirectional electromagnetic wave. Consequently, the directivity of coaxial coupler is substantially increased.

Referring now to FIGURE 2, which is a top view in cross section of the coaxial coupler 10 of FIGURE 1 including two distributed compensation structures according to other embodiments of this invention, there is shown a distributed compensation structure 32 comprising a dielectric slab 34 having a relative permittivity greater than that of the surrounding medium, which in this case is air.

Dielectric slab 34 is symmetrically positioned on the lower ground plane 18 between the diverging portions 36 and 38 of primary and auxiliary conductors 12 and 14. The basal plane of dielectric slab 34 and the parallel reference plane defined between the adjacent edges of the diverging portions 36 and 38 of primary and auxiliary conductors 12 and 14 are congruous. However, the distance between the outer edges of dielectric slab 34 and the projection 28 of the reference plane should not generally exceed one-tenth of the diameter or width of the primary or auxiliary conductors 12 or 14. Though the dimensions of dielectric slab 34 are not critical, preferably, it has a transverse cross section 36 (shown in FIGURE 3) which decreases in thickness away from the center. Additionally, the thickest portion of the dielectric slab 34 should not extend substantially past the plane containing the lower edges of primary and auxiliary conductors 12 and 14.

Dielectric slab 34 decreases the mutual capacitance and,

hence, the electric field coupling between the diverging portions 36 and 38 of the primary and auxiliary conductors 12 and 14. This compensates for the otherwise greater reduction in the magnetic field coupling relative to the electric field coupling caused by the divergence of primary and auxiliary conductors 12 and 14 and increases the directivity of coaxial coupler 10. For example, a four to eight kilomegacycle coaxial coupler which normally has a best directivity ranging from fifteen to seventeen decibels may achieve a best directivity ranging from twenty-three to twenty-six decibels with a pair of dielectric slabs 34. One dielectric slab 34 is positioned on the lower ground plane 18 between the diverging portions 36 and 38 of primary and auxiliary conductors 12 and 14; the other is similarly positioned between the diverging portions 40 and 42 of primary and auxiliary conductors 12 and 14. Though substantial improvement in directivity is obtained with a single pair of dielectric slabs 34, additional improvement may be achieved by similarly positioning a pair of dielectric slabs 34 on the upper ground plane 16 (shown in FIGURE 1).

Instead of one dielectric slab 34, a distributed compensation structure 44 comprising a plurality of dielectric posts 46 may be used as shown between the diverging portions 40 and 42 of primary and auxiliary conductors 12 and 14. The greatest concentration of dielectric posts 46 is located in the region nearest the point of divergence of portions 40 and 42 of primary and auxiliary conductors 12 and 14 since this is the region of greatest coupling between the diverging portions 40 and 42.

Referring to FIGURE 4, there is shown a perspective view of a strip line coupler 48 partially broken to reveal its construction. A pair of parallel conductive ground planes 50 and 52 and a conductive side wall 54 form the outer conductor. The primary and auxiliary strip conductors 56 and 58 are positioned in the same plane in transverse electric and magnetic field coupling proximity within this outer conductor. Primary and auxiliary strip conductors 56 and 58 are supported in a dielectric slab 60 which occupies the space between ground planes 50 and 52. Primary strip conductor 56 extends longitudinally through dielectric slab 60 and is connected to the inner conductors of coaxial connectors 62 mounted in opposite end sections of the outer conductor. A central portion of the auxiliary strip conductor 58 is positioned adjacent and parallel to the primary strip conductor 56. The exposed end portion of the auxiliary strip conductor 58 diverges from the primary strip conductor 56 and is connected to the inner conductor of a coaxial connector 62 mounted in one side of the outer conductor. The unexposed end portion of the auxiliary strip conductor 58 also diverges from the pirmary strip conductor 56 and is connected through a load 64 (shown in FIG- URE 5) to ground plane 52.

A distributed compensation structure comprising a cavity 66 in the dielectric slab 60 is disposed in the coupling region between the exposed and diverging portions of the primary and auxiliary strip conductors 56 and 58. A similar cavity 66 in the dielectric slab 60 is disposed in the coupling region between the unexposed and diverging portions of the primary and auxiliary strip conductors 56 and 58. The volume of these cavities 66 is limited only by the contour of the reference plane defined between the adjacent edges of the diverging portions of primary and secondary strip conductors 56 and 58 and the separation between the parallel conductive ground planes 50 and 52. However, the cavities 66 should not generally extend to the adjacent edges of the primary and secondary strip conductors 56 and 58. These cavities 66 decrease the mutual capacitance and, hence, the electric field coupling between the diverging portions of the primary and secondary strip conductors 56 and 58. This compensates for the more significant reduction in the magnetic field coupling relative to the electrical field coupling between the diverging portions of the primary and secondary strip conductors 56 and 58 and increases the directivity of the strip line coupler 48.

Alternatively, the compensation structures may comprise a plurality of holes 68 in the dielectric slab 60 as shown in FIGURE 5, which is a top view in cross section of the strip line coupler of FIGURE 4 according to this embodiment of my invention. The greatest concentration of holes 60 is located in the regions nearest the points of divergence of primary and auxiliary strip conductors 56 and 58 since these are the regions of greatest coupling between the diverging portions of the primary and auxiliary strip conductors 56 and 58.

I claim:

1. A TEM mode coupler for coupling energy over a broad frequency range with high directivity comprising:

a first electromagnetic wave energy transmission path including a first conductor;

a second electromagnetic wave energy transmission path including a second conductor;

said first and second conductors being positioned in transverse electric and magnetic field coupling proximity with a finite spacing between the adjacent edges of said conductors;

said second conductor including a portion which diverges from said first conductor; and

a compensation structure for altering one of the electric and magnetic fields coupling between the diverging portions of said first and second conductors to improve the directivity of coupling between the first and second conductors;

said compensation structure including one of a dielectric-filled cavity and a dielectric element, said one of the dielectric-filled cavity and the dielectric element including a directivity-improving portion in the region of divergence of said first and second conductors.

2. A TEM mode coupler for coupling energy over a broad frequency range with high directivity comprising:

a first electromagnetic wave energy transmission path including a first conductor positioned near a conductive ground plane;

a second electromagnetic wave energy transmission path including a second conductor positioned near said conductive ground plane;

said first and second conductors being positioned in the same plane in transverse electric and magnetic field coupling proximity with a finite spacing between the adjacent edges of said conductors;

said second conductor including a portion which diverges from said first conductor; and

a compensation structure for altering one of the electric and magnetic fields coupling between the diverging portions of said first and second conductors to improve the directivity of coupling between the first and second conductors;

said compensation structure comprising at least one directivity-improving cavity in the inner surface of said ground plane in the coupling region between the diverging portions of said first and second conductors.

3. A TEM rnode coupler for coupling energy over a broad frequency range with high directivity comprising:

a first electromagnetic wave energy transmission path including a first conductor positioned intermediate a pair of conductive ground planes;

a second electromagnetic wave energy transmission path including a second conductor positioned intermediate said pair of conductive ground planes;

said first and second conductors being positioned in the same plane in transverse electric and magnetic field coupling proximity with a finite spacing between the adjacent edges of said conductors;

said second conductor including a portion which diverges from said first conductor; and

a distributed compensation structure for altering one of the electric and magnetic fields coupling between the diverging portions of said first and second conductors to improve the directivity of coupling between the first and second conductors;

said distributed compensation structure comprising a plurality of elongated directivitydmproviug cavities in the inner surface of one of said ground planes in the coupling region between the diverging portions of said first and second conductors;

said plurality of cavities bearing a transverse relation ship to the diverging portions of said first and second conductors.

4. A TEM mode coupler as in claim 3 wherein said plurality of cavities in the inner surface of said one ground plane is substantially symmetrical with respect to the projection thereon of a parallel reference plane defined between the adjacent edges of the diverging portions of said first and second conductors.

5. A TEM mode coupler for coupling energy over a broad frequency range with high directivity comprising:

a first electromagnetic wave energy transmission path including a first conductor positioned near a conductive ground plane;

a second electromagnetic wave energy transmission path including a second conductor positioned near said conductive ground plane;

said first and second conductors being positioned in the same plane in transverse electric and magnetic field coupling proximity with a finite spacing between the adjacent edges of said codnuctors;

said second codnuctor including a portion which diverges from said first conductor; and

a compensation structure for altering one of the electric and magnetic fields coupling between the diverging portions of 'said first and second conductors to improve the directivity of coupling between the first and second conductors;

said compensation structure comprising at least one directivity-improving dielectric element having a relative permittivity greater than that of the surrounding medium;

said dielecrtric element being positioned in the coupling region between the diverging portions of said first and second conductors.

6. A TEM mode coupler as in claim 5 wherein said dielectric element comprises a plurality of dielectric posts positioned on the inner surface of said ground plane.

7. A TEM mode coupler as in claim 5 wherein said dielectric element comprises a slab of dielectric material positioned on the inner surface of said ground plane.

8. A TEM mode coupler as in claim 7 wherein the basal plane of said slab of dielectric material and a reference plane defined between the adjacent edges of the diverging portions of said first and second conductors are substantially congruous;

said basal plane being positioned on the inner surface of said ground plane in substantial symmetry with respect to the projection thereon of said reference plane;

said slab of dielectric material having a thickness which increases towards the center line thereof.

9. A TEM mode coupler for coupling energy over a broad frequency range with high directivity comprising:

a first electromagnetic wave energy transmission path including a first conductor positioned intermediate a pair of parallel conductive ground planes;

a second electromagnetic wave energy transmission path including a second conductor positioned intermediate said pair of parallel conductive ground planes;

said first and second conductors being positioned in the same plane in transverse electric and magnetic field coupling proximity with a finite spacing between the adjacent edges of said conductors;

said second conductor including a portion which diverges from said first conductor; and

a slab of dielectric material filling the space between said pair of parallel conductive ground planes;

said dielectric slab including a distributed compensation structure for altering one of the electric and magnetic fields coupling between the diverging portions of said first and second conductors to improve the directivity of coupling between the first and second conductors;

said distributed compensation network comprising at least one directivity-improving cavity in said slab of dielectric material;

said cavity being disposed intermediate said pair of parallel conductive ground planes in the coupling region between the diverging portions of said first and second conductors.

10. A TEM mode coupler as in claim 9 wherein said cavity occupies a substantial portion of the coupling region between the diverging portions of said first and second conductors.

11. A TEM mode coupler as in claim 9 wherein said cavity comprises a plurality of holes with the concentration of said holes decreasing away from the point of divergence of said first and second conductors.

7 12. A TEM coupler for coupling energy over a broad 'equency range with high directivity comprising:

a first electromagnetic wave energy transmission path including a first conductor supported by a dielectric slab near a conductive ground plane.

a second electromagnetic wave energy transmission path including a second conductor supported by said dielectric slab near said conductive ground plane.

said first and second conductors being positioned in the same plane in transverse electric and magnetic field coupling proximity with a finite spacing between the adjacent edges of said conductors;

said second conductor including a portion which diverges from said first conductor; and

said dielectric slab including at least one directivityimproving cavity for altering one of the electric and magnetic fields coupling between the diverging portions of said first and second conductors to improve the directivity of coupling between the first and second conductors;

said cavity being disposed in the coupling region between the adjacent edges of the diverging portions of said first and second conductors.

References Cited by the Examiner UNITED STATES PATENTS 2,951,218 8/1960 Arditi 333-24 X HERMAN KARL SAALBACH, Primary Examiner.

15 M. NUSSBAUM, Assistant Examiner. 

1. A TEM MODE COUPLER FOR COUPLING ENERGY OVER A BROAD FREQUENCY RANGE WITH HIGH DIRECTIVITY COMPRISING: A FIRST ELECTROMAGNETIC WAVE ENERGY TRANSMISSION PATH INCLUDING A FIRST CONDUCTOR; A SECOND ELECTROMAGNETIC WAVE ENERGY TRANSMISSION PATH INCLUDING A SECOND CONDUCTOR; SAID FIRST AND SECOND CONDUCTORS BEING POSITIONED IN TRANSVERSE ELECTRIC AND MAGNETIC FIELD COUPLING PROXIMITY WITH A FINITE SPACING BETWEEN THE ADJACENT EDGES OF SAID CONDUCTORS; SAID SECOND CONDUCTOR INCLUDING A PORTION WHICH DIVERGES FROM SAID FIRST CONDUCTOR; AND A COMPENSATION STRUCTURE FOR ALTERING ONE OF THE ELECTRIC AND MAGNETIC FIELDS COUPLING BETWEEN THE DIVERGING PORTIONS OF SAID FIRST AND SECOND CONDUCTORS TO IMPROVE THE DIRECTIVITY OF COUPLING BETWEEN THE FIRST AND SECOND CONDUCTORS; SAID COMPENSATION STRUCTURE INCLUDING ONE OF A DIELECTRIC-FILLED CAVITY AND A DIELECTRIC ELEMENT, SAID ONE OF THE DIELECTRIC-FILLED CAVITY AND THE DIELECTRIC ELEMENT INCLUDING A DIRECTIVITY-IMPROVING PORTION IN THE REGION OF DIVERGENCE OF SAID FIRST AND SECOND CONDUCTORS. 